1. Field of the Invention
This invention relates to a developer used for developing electrostatic latent images, an image forming method, and a multicolor image forming method in an electrophotographic process or an electrostatic recording process.
2. Description of the Related Art
In an electrophotographic process, electrostatic latent images formed on a photoreceptor are developed using a toner containing a colorant, the developed toner images are transferred to transfer sheets, and the toner images are fixed using a heat roller, to prepare images. On the other hand, the photoreceptor is cleaned to remove toner remaining on the photoreceptor to prepare for forming the next image. Dry developers used for such an electrophotographic process are roughly classified into one-component developers using only a toner produced by dispersing a colorant in a binder resin and two-component developers produced by mixing the toner with a carrier.
Incidentally, in recent as advances in compact, high-speed, long-lived copy machines, high-speed photoreceptor and a developer carrying substrate having a smaller diameter have been needed, due to the cause of increasing stress to the developer. It is therefore necessary that the developer be superior in flow, resistance to caking, fixability, charge property, and cleaning property. Especially, fine inorganic powder is often added to the surface of a toner so as to promote flow and resistance to caking.
However, fine inorganic powder has a considerable influence on the charge property of the toner. For example, because fine silica powder generally used has negative polarity, the fine silica powder increases the charge amount of the negative-polarity toner excessively, especially at a low temperature and low humidity. On the other hand, because fine silica powder takes in water under conditions of high temperature and high humidity, it acquires high electroconductivity and hence its charge property is reduced. Charging properties differ in both circumstances and result in the failure of density reproducibility, fogging on the background and copying failure caused by dirtying of the machine. The characteristics of the toner are also influenced by the dispersing condition of fine inorganic powder on the toner. When fine inorganic powder is dispersed unevenly on the toner, sufficient flow and resistance to caking cannot be attained. There are cases in which, in a cleaning process, toner remaining on the photoreceptor is removed insufficiently and the toner adheres to the photoreceptor, resulting in the occurrence of sunspot-like image failures and dropout.
In order to solve these problems, methods are proposed in which fine inorganic powder is surface-treated. For example, Japanese Patent Applications Laid-Open (JP-A) Nos. 46(1971)-5782, 48(1973)-47345, and 48(1973)-47346 disclose a method, in which the surface of fine silica powder is processed by a hydrophobic treatment for the purpose of preventing the adverse influence of water and the like. However, the use of such a fine inorganic powder alone cannot have sufficient effect in the prevention of the fluctuation of the charge property caused by environmental change, in the prevention of lowering of charging stability caused by long-term stress, and in the prevention of secondary damage to the photoreceptor.
There have been known methods to decrease the negative charge of fine silica powder added to the surface of a toner. These methods include a method in which fine silica powder is surface-treated with an amino-modified silicone oil (see Japanese Patent Application Laid-Open (JP-A) No. 64(1989)-73354), and a method in which fine silica powder is surface-treated with aminosilane and/or amino-modified silicone oil (see Japanese Patent Application Laid-Open (JP-A) No. 1(1989)-237561). These treatments using amino compounds ensure restriction on the excess rise in the charge of negatively charged toner, but do not have sufficient effect in improving the fluctuation of the charge property of fine silica powder, which property is caused by environmental change. More specifically, using these treatments, fine silica powder can only be slightly suppressed from being excessively negatively charged after long-term operation of the apparatus at low temperature and low humidity. However, because the charge of fine silica powder is also neutralized by these treatments after long-term operation of an apparatus at a high temperature and high humidity, the charge fluctuation caused by environmental change has not been able to be improved yet. When a polyester resin or an epoxy resin is used as the binder resin for the toner, charging properties differ greatly between conditions of high temperature and high humidity and conditions of low temperature and low humidity.
From the above points of view, a method for adding an inorganic compound other than silica to the surface of a toner has been investigated. For example, a method in which an inorganic oxide such as titanium oxide having a low charge property or the like is added to the surface of a toner has been proposed (see Japanese Patent Application Laid-Open (JP-A) Nos. 58(1983)-216252, 60(1985)-123862, and 60(1985)-238847).
However, because titanium oxide is produced by purifying and baking titanium oxyhydrate (TiO(OH).sub.2) obtained by treating an ilmenite ore using a sulfuric acid process (wet process), it is only natural that aggregated particles occur due to dehydration and condensation in the product obtained using this process. It is not easy to disperse aggregated particles again using conventional techniques. Accordingly, crystallized titanium oxide (rutile: specific gravity of 4.2; anatase: specific gravity of 3.9) separated from the system of reaction as fine powder includes secondary and tertiary aggregated particles, so that a toner composition containing, as an external additive, titanium oxide is greatly inferior in improving flow compared to a toner composition containing, as an external additive, silica. Recently, there have been increasing numbers of requests in the market for improvement of high-quality images, especially high-quality color images and hence attempts have been made to produce a toner with a smaller particle diameter to achieve a high-quality image. However, as toner particles become finer, the adhering force between particles increases, whereby the flow of toner is further adversely affected. This phenomenon is greatly aggravated when titanium oxide is used as an external additive which is added to the surface of the toner.
Because of this, an attempt has been made to add both hydrophobic titanium oxide and hydrophobic silica to the surface of the toner, in order to improve the flow and prevent fluctuation in the charge property caused by environmental change (see Japanese Patent Application Laid-Open (JP-A) No. 60(1985)-136755). Using this process, the drawbacks of hydrophobic titanium oxide and hydrophobic silica can be temporarily eliminated. However, it is difficult to control the dispersing conditions of both on the surface of the toner in a stable manner and hence the characteristics of either hydrophobic titanium oxide or hydrophobic silica tends to be exhibited. Accordingly, it is difficult to cover the drawbacks of both together over the long term.
As another method for achieving both improvement in flow and the prevention of fluctuation in the charge property caused by environmental change, a method in which hydrophobic amorphous titanium oxide is added to the surface of a toner has been proposed (see Japanese Patent Application Laid-Open (JP-A) Nos. 5(1993)-204183 and 5(1993)-72797). The above amorphous titanium oxide can be prepared by hydrolyzing a metal alkoxide or a metal halide using a CVD process (see Chemical Engineering Articles Vol. 18, No. 3, 303-307, 1992).
However, because amorphous titanium oxide contains a large amount of adsorbed water within particles, it tends to adhere to the photoreceptor, so that amorphous titanium oxide remains on the photoreceptor in the transferring process. This hard amorphous titanium oxide residue may damage the photoreceptor in the cleaning process, or may cause dropout, if it cannot be removed from the photoreceptor by cleaning.
As methods for producing titanium oxide through the wet process, a method has been proposed wherein a coupling agent is hydrolyzed in an aqueous medium and the surface of the titanium oxide is treated with a hydrolyzed coupling agent to obtain hydrophobic titanium oxide which is suppressed from aggregating, followed by adding hydrophobic titanium oxide to a toner (see Japanese Patent Application Laid-Open (JP-A) No. 5(1993)-188633).
The hydrophobic titanium oxide produced in this method using a silane coupling agent improves the charging property of a negatively charged toner and the flow of toner in the early stages. However, the treating agent (silane coupling agent) for titanium oxide, which is added to the surface of the toner, tends to peel off the surface of the toner due to the collision of the toner with a carrier or the friction of the toner with a blade or a sleeve which are caused by agitation, whereby the charging property of the toner fluctuates greatly. Specifically, this method causes the life of the developer to be greatly shortened. Although the mechanism behind this phenomenon is not clear, it is hypothesized that this phenomenon is caused for the following reason: Specifically, even though titanium oxide reacts with the silane coupling agent, the binding force therebetween is significantly lower than the binding force, for example, between silica and aminosilane, because titanium oxide is lower in alkalinity. Also, titanium oxide has a few hydroxyl groups which are activated and there are limitations to the amount of silane compound used for the treatment (this amount contributes to the charge property), demonstrating the problem that a highly negative polarity cannot be obtained even in the early stages.
On the other hand, it is generally known that a titanium coupling agent strongly binds with titanium oxide. However, in the above method for treating titanium oxide by utilizing the wet process, it is necessary that a treating agent be dissolved or dispersed in water. Therefore, it is difficult to use a titanium coupling agent because titanium coupling agents have long molecular chains and hence most of these are insoluble in water. Also, only one titanium coupling agent which is soluble in water is a titanium coupling agent containing an amino group. This coupling agent has the capability of providing a positive charge, which makes it unsuitable for a treating agent used for an external additive for a negatively charged toner. Also, in titanium oxide produced in this method, the amount of secondary and tertiary aggregated particles is reduced, so that titanium oxide remains on a toner as primary particles, whereby the flow of toner is improved. However, the adhering condition of the titanium oxide to the toner is greatly changed from that at an early stage due to long-term stress. Therefore, the amount of charge of a toner used to supplement a developer during the operation of an apparatus is different from that of a toner included in developer before the adding of new toner, leading to inferior charging which is thought to be caused by the difference between the amounts of charge of toners. This causes the dirtying of a machine, fogging on the background, and the like.
As a method for forming full-color electrophotographic images in which full colors are rendered using toners of three colors of yellow, magenta, and cyan, or four colors consisting of those three colors and black, for example, there is a method in which a process including charging, exposure, and developing is repeated for each color to superimpose toners on a photoreceptor, then toner images are transferred to a transfer media, and the full-color images on the transfer media are fixed.
It is strongly desired that color toners used in such a method for forming color images not be mixed in individual developing steps and that they have transparency so that the colors of toners, especially underlying toners, are not damaged when these toners are superimposed.
Among these, it is known that the transparency of a color toner decreases as the amount of an external additive added to the surface of the toner increases. Transparency is especially remarkably reduced when titanium oxide is used as the external additive. Specifically, if the amount of titanium oxide is increased to improve the flow of toner, the transparency of the toner deteriorates extremely.
On the other hand, there has been an attempt to improve a carrier. For example, a method for reproducing high-quality images accurately, especially half tone, black solid, and printed characters, by controlling the volumetric specific resistance of a carrier is proposed. (see Japanese Patent Applications Laid-Open (JP-A) Nos. 56(1981)-125751 and 62(1987)-267766, and Japanese Patent Application Publication (JP-B) No. 7(1995)-120086).
In all of these methods, the resistance of the carrier is controlled by selecting a suitable material for a coating layer of the carrier or by controlling the amount of the coating layer. Using these measures, the desired volumetric specific resistance and a high-quality images can be obtained at an early stage. However, peeling of the carrier coating layer and the like caused by the stress generated in a developing unit occur, whereby the volumetric specific resistance of the carrier is greatly changed. Therefore, it is difficult to maintain high-quality images over the long term.
A method for controlling the volumetric specific resistance of a carrier by adding carbon black to the coating layer of the carrier is also proposed (see Japanese Patent Application Laid-Open (JP-A) No. 4(1992)-40471).
This method is useful for suppressing the fluctuation of the volumetric specific resistance of the carrier caused by the peeling of the coating layer. However, the external additive added to the surface of the toner or toner components adhere to the carrier, whereby the volumetric specific resistance of the carrier is changed. Therefore, it is still difficult to supply high-quality images over the long term.